Research On Spotlight Synthetic Aperture Radar Imaging Technology

Spotlight mode which is an effective means to gain high- resolution in the azimuth direction is one of the mature working modes of the synthetic aperture radar. During the flight of the platform,spotlight mode constantly steers the antenna direction and the radar beam illuminates the same scene center from beginning to end. Thus,a much longer synthetic aperture than that of the stripmap SAR is available. The azimuth resolution of spotlight SAR is only related to the wavelength and the turn angle,while not restricted by the antenna size in the azimuth direction. Thereby,compared with the SAR system with fixed beam direction,higher resolution in the azimuth direction is acquired. Spotlight SAR working at the squint mode expands its scope of application and improves the flexibility of the system,yet the difficulty of the echo focusing is increased.The dissertation studies the imaging technology based upon the model of squint spotlight SAR. The study includes: the simulation of the squint spotlight SAR echo,squint chirp scaling algorithm,frequency scaling algorithm,nonlinear frequency scaling algorithm.Firstly,the geometric model of the squint spotlight SAR is built. And the echo characteristics, azimuth frequency history, range migration, space-varying of the phase history and the focus depth are analyzed in detail. Furthermore,the difference between spotlight SAR and stripmap SAR is focused on the analysis. And the detail derivation process of the key parameters is given.Secondly, the squint chirp scaling algorithm is studied and utilized in the processing of the spotlight SAR echo data. The simulation results show that under the condition of small squint angle or small scene, the imaging results of squint chirp scaling algorithm is favorable. Since the space-varying of the second range compression and the effect of high order phase term are ignored in the algorithm, the imaging results under the condition of large squint angle or large scene are unsatisfactory. Moreover, the squint chirp scaling algorithm is not suitable for the spotlight SAR echo after dechirp processing.Thirdly, the frequency scaling algorithm is studied and the principle of frequency scaling is emphasized. The simulation results show that under the condition of small squint angle or small scene, the imaging results of frequency scaling algorithm is good. Meanwhile, the limitation of frequency scaling algorithm is analyzed in detail. For the ignorance of the space- varying of the second range compression,while the squint angle increasing or to the point at the edge of large scene,the frequency scaling algorithm is not suitable any more.Finally, the nonlinear frequency scaling algorithm is studied. The detail derivation process of the whole algorithm is given. The principle and specific process of nonlinear frequency scaling is emphasized. The simulation results show that the nonlinear frequency scaling algorithm works well for the large squint model or large scene and the imaging results are acceptable.